1. Field of the Invention
The present invention relates to a fixed-focus imaging lens which can be suitably mounted on a small-sized imaging apparatus such as a digital camera using an image device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), a camera using a silver film, etc.
2. Description of Background Art
In recent years, with the popularization of personal computers over general homes or the like, digital still cameras (hereinafter referred to as “digital cameras” simply) capable of inputting image information of photographed scenes, persons, etc. into personal computers have been coming into wide use rapidly. With the sophistication of cellular phones, module cameras (portable module cameras) for inputting images have been often mounted on cellular phones.
In such an imaging apparatus, an imaging device such as a CCD or a CMOS is used. With recent development in miniaturization of imaging devices, such an imaging apparatus as a whole has been highly miniaturized. With higher pixel counts in imaging devices, higher resolution and higher performance have been developed.
For example, imaging lenses for use in such a miniaturized imaging apparatus have been disclosed in the following patent documents. JP-A-10-48516 and JP-A-2002-221659 disclose imaging lenses with three-lens configurations respectively. JP-A-2004-302057, JP-A-2005-24581, JP-A-2005-4027 and JP-A-2005-4028 disclose imaging lenses with four-lens configurations respectively. The imaging lens disclosed in JP-A-2004-302057 has a stop disposed between a second lens and a third lens from the object side. The imaging lens disclosed in JP-A-2005-24581 has a stop disposed the most closely to the object. The imaging lens disclosed in each of JP-A-2005-4027 and JP-A-2005-4028 has a stop disposed the most closely to the object or between a first lens and a second lens from the object side.
As described above, with the miniaturization and the higher pixel counts in recent imaging devices, particularly imaging lenses for digital cameras are requested to have higher resolution performance and to have a compact configuration. On the other hand, imaging lenses for portable module cameras have been heretofore chiefly requested to be low in cost and compact in configuration. Recently, also portable module cameras have showed a tendency to make their imaging devices higher in pixel counts. Thus, the imaging devices in the portable module cameras have been requested to have higher performance.
It is therefore desired to develop a wide variety of lenses comprehensively improved in cost, imaging performance and compact configuration. For example, it is desired to develop low-cost and high-performance imaging lens having compactness secured to be high enough to be mounted even in a portable module camera, and having performance high enough to be mounted even in a digital camera.
To meet these requests, for example, it is considered that the number of lenses is three or four in order to secure compactness and low cost, and aspheric surfaces are used aggressively in order to secure high performance. In this case, the aspheric surfaces contribute to compactness and high performance. However, the aspheric surfaces are disadvantageous in terms of manufacturing efficiency, and the cost is increased easily. It is therefore desired to take the manufacturing efficiency into consideration when the aspheric surfaces are used. The lenses according to the aforementioned patent documents have a three-lens or four-lens configuration using aspheric surfaces. However, they are insufficient, for example, in terms of compatibility between imaging performance and compactness.